THE NATURAL HISTORY OF MAN 73 



the plica semilunaris, which is relatively larger during prenatal 

 life. It is believed to be a reduced third eyelid, such as is 

 regularly found in amphibia and birds, by which the eye is 

 closed. 



Man possesses muscles which are arranged to move the ear 

 either as a whole or in part, but he has entirely lost the con- 

 trol of them or only exceptionally retains partial control. 

 Probably when man assumed the erect posture with eyes look- 

 ing forward the head became more movable and rendered 

 unnecessary the mobility of the ear with the resulting loss of 

 function in its muscles. At one period during prenatal de- 

 velopment the human ear is pointed and resembles the ear of 

 certain monkeys. This point persists in many people as a 

 projection on the rolled or unrolled rim (helix) of the ear. 

 It is known as Darwin's tubercle and is the vestigial remains of 

 man's ancestral ear point. 



Near the junction of the large and small intestine in man 

 (Fig. 26) there is a narrow, blind process, about three and a 

 half inches in length, known as the vermiform appendix. The 

 appendix in man is a vestigial structure and represents the func- 

 tionless, shriveled, terminal remains of the caecum, the blind 

 beginning of the large intestine. In an herbivorous animal, 

 the caecum is a large, nutritive organ of great importance. In 

 carnivorous animals, the caecum is reduced. The reduction of 

 the terminal portion of the caecum to form an appendix occurs 

 only in man (Fig. 26), the anthropoid apes, and some rodents. 

 The frequent pathological condition of the appendix in man 

 has given rise to the aphorism "that vestigial structures are 

 particularly prone to disease." 



There are various structures in the body which, although 

 not vestigial, give evidence of a retrogression. Such a condi- 

 tion is to be noted in certain of the sense organs. The olfac- 

 tory organ, for example, which is of the greatest importance 

 to the lower mammals and other classes of the vertebrates, 



